理解Android系统的进程间通信原理------RPC机制

理解Android系统中的轻量级解决方案RPC的原理,需要先回顾一下JAVA中的RMI(Remote Method Invocation)这个易于使用的纯JAVA方案(用来实现分布式应用)。有关RMI的相关知识,可以通过下图来归纳:

Android中的RPC也是参考了JAVA中的RMI方案,这里我们再详细了解一下RPC的实现过程。
Android中的RPC机制是为了实现一个进程使用另一个进程中的远程对象,它使用了Android自己的AIDL(接口定义语言),使用户很方便地定义出一个接口作为规范,通过一个远程Service为代理 ,客户端在绑定该远程Service过程中获取远程对象,进而使用该对象。可参考下图所示:

补充:RPC的另一个目的是对客户端只声明接口及方法,隐藏掉具体实现类,供客户端直接获取此接口实例。
实例代码:
实例一:通过Service来远程调用一个接口子类的函数方法
功能描述:在MainActivity中通过绑定MyService服务类,来远程调用MyPlayer(实现了IPlayer接口)的方法过程。需要定义一个IPlayer.aidl文件,ADT工具会自动生成一个IPlayer接口类,然后再由MyPlayer继承IPlayer接口类中的静态内部抽象类,实现接口方法,进而供其它应用程序远程调用。(在本例中为了方便,MainActivity与MyService类同处一个应用程序中,实现运用时,可以不在同一个应用程序中,只要有权限访问MyService服务,就能得到IPlayer接口,进而执行该接口实例方法)
程序清单:IPlayer.aidl

package com.magc.rpc;
 
interface IPlayer
 {
     void setName(String name);
     void addFile(String f_name);
     String ToString();
 }
 程序清单:IPlayer.java (ADT根据上面IPlayer.aidl文件自动生成,不能编辑该文件)
 /*
 * This file is auto-generated.  DO NOT MODIFY.
 * Original file: F:\\work\\Android_App\\MyRPCService\\src\\com\\magc\\rpc\\IPlayer.aidl
 */
 package com.magc.rpc;
 public interface IPlayer extends android.os.IInterface
 {
 /** Local-side IPC implementation stub class. */
 public static abstract class Stub extends android.os.Binder implements com.magc.rpc.IPlayer
 {
 private static final java.lang.String DESCRIPTOR = "com.magc.rpc.IPlayer";
 /** Construct the stub at attach it to the interface. */
 public Stub()
 {
 this.attachInterface(this, DESCRIPTOR);
 }
 /**
 * Cast an IBinder object into an com.magc.rpc.IPlayer interface,
 * generating a proxy if needed.
 */
 public static com.magc.rpc.IPlayer asInterface(android.os.IBinder obj)
 {
 if ((obj==null)) {
   return null;
 }
 android.os.IInterface iin = (android.os.IInterface)obj.queryLocalInterface(DESCRIPTOR);
 if (((iin!=null)&&(iin instanceof com.magc.rpc.IPlayer))) {
   return ((com.magc.rpc.IPlayer)iin);
 }
   return new com.magc.rpc.IPlayer.Stub.Proxy(obj);
 }
 public android.os.IBinder asBinder()
 {
   return this;
 }
 @Override public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException
 {
 switch (code)
 {
 case INTERFACE_TRANSACTION:
 {
   reply.writeString(DESCRIPTOR);
   return true;
 }
 case TRANSACTION_setName:
 {
   data.enforceInterface(DESCRIPTOR);
   java.lang.String _arg0;
   _arg0 = data.readString();
   this.setName(_arg0);
   reply.writeNoException();
   return true;
 }
 case TRANSACTION_addFile:
 {
   data.enforceInterface(DESCRIPTOR);
   java.lang.String _arg0;
   _arg0 = data.readString();
   this.addFile(_arg0);
   reply.writeNoException();
   return true;
 }
 case TRANSACTION_ToString:
 {
    data.enforceInterface(DESCRIPTOR);
   java.lang.String _result = this.ToString();
   reply.writeNoException();
   reply.writeString(_result);
   return true;
 }
 }
 return super.onTransact(code, data, reply, flags);
 }
 private static class Proxy implements com.magc.rpc.IPlayer
 {
 private android.os.IBinder mRemote;
 Proxy(android.os.IBinder remote)
 {
   mRemote = remote;
 }
 public android.os.IBinder asBinder()
 {
 return mRemote;
 }
 public java.lang.String getInterfaceDescriptor()
 {
 return DESCRIPTOR;
 }
 public void setName(java.lang.String name) throws android.os.RemoteException
 {
   android.os.Parcel _data = android.os.Parcel.obtain();
   android.os.Parcel _reply = android.os.Parcel.obtain();
 try {
   _data.writeInterfaceToken(DESCRIPTOR);
   _data.writeString(name);
   mRemote.transact(Stub.TRANSACTION_setName, _data, _reply, 0);
   _reply.readException();
 }
 finally {
   _reply.recycle();
   _data.recycle();
 }
 }
 public void addFile(java.lang.String f_name) throws android.os.RemoteException
 {
   android.os.Parcel _data = android.os.Parcel.obtain();
   android.os.Parcel _reply = android.os.Parcel.obtain();
 try {
   _data.writeInterfaceToken(DESCRIPTOR);
   _data.writeString(f_name);
   mRemote.transact(Stub.TRANSACTION_addFile, _data, _reply, 0);
   _reply.readException();
 }
 finally {
   _reply.recycle();
   _data.recycle();
 }
 }
 public java.lang.String ToString() throws android.os.RemoteException
 {
   android.os.Parcel _data = android.os.Parcel.obtain();
   android.os.Parcel _reply = android.os.Parcel.obtain();
   java.lang.String _result;
 try {
     _data.writeInterfaceToken(DESCRIPTOR);
     mRemote.transact(Stub.TRANSACTION_ToString, _data, _reply, 0);
     _reply.readException();
     _result = _reply.readString();
 }
 finally {
     _reply.recycle();
     _data.recycle();
 }
   return _result;
 }
 }
   static final int TRANSACTION_setName = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
   static final int TRANSACTION_addFile = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
   static final int TRANSACTION_ToString = (android.os.IBinder.FIRST_CALL_TRANSACTION + 2);
 }
   public void setName(java.lang.String name) throws android.os.RemoteException;
   public void addFile(java.lang.String f_name) throws android.os.RemoteException;
   public java.lang.String ToString() throws android.os.RemoteException;
 }
 程序清单:MyPlayer.java  (实现IPlayer的静态内部抽象类Stub)
 package com.magc.rpc;
 
import android.os.RemoteException;
 import android.util.Log; 
import com.magc.rpc.IPlayer.Stub;
 /**
 * 
* @author magc
 * 实现IPlayer接口类中的静态内部抽象类,即实现IPlayer接口方法
* 将来供其它应用程序远程调用执行方法
*/
 public class MyPlayer extends Stub {
 
    private String name="";
     @Override
     public void addFile(String fName) throws RemoteException {
         
         System.out.println("add file ...");
     }
 
    @Override
     public void setName(String name) throws RemoteException {
 
        this.name = name;
         Log.i("magc", "setName--"+name);
     }
     
    public String ToString()
     {
         String str = "MyPlayer--"+name;
         Log.i("magc", "MyPlayer--"+name);
         return str;
     }
 
}
 程序清单:MyService.java (一个Service类,供其它程序来远程绑定,返回IPlayer接口)
 package com.magc.rpc;
 
import com.magc.rpc.IPlayer.Stub; 
import android.app.Service;
 import android.content.Intent;
 import android.os.IBinder;
 
/**
 * 
* @author magc
 * 此服务类作为一个代理角色,供其它应用程序绑定,并返回接口实例
* 
* 可看作是代理模式的应用
*/
 public class MyService extends Service {
 
    private Stub player = new MyPlayer();
     @Override
     public IBinder onBind(Intent arg0) {
         return player;
     }
 
    @Override
     public void onCreate() {
         super.onCreate();
     }
 
}
 程序清单:MainActivity.java (作为客户端远程调用IPlayer接口方法)
 package com.magc.rpc;
 
import android.app.Activity;
 import android.content.ComponentName;
 import android.content.Intent;
 import android.content.ServiceConnection;
 import android.os.Bundle;
 import android.os.IBinder;
 import android.os.RemoteException;
 import android.util.Log;
 /**
 * 
* @author magc
 * 作为一个客户端通过绑定MyService服务,实现远程调用IPlayer接口方法
* 
*/
 public class MainActivity extends Activity {
     private  String ACTION="com.magc.rpc.action.MYSERVICE";
     private IPlayer player;
     /** Called when the activity is first created. */
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.main);
         Intent intent = new Intent();
         intent.setAction(ACTION);
         //绑定MyService服务
         bindService(intent, conn, BIND_AUTO_CREATE);
                 
     }
     private ServiceConnection conn = new ServiceConnection() {
         
         @Override
         public void onServiceDisconnected(ComponentName name) {
             
         }      
         /**
          * 绑定MyService服务后,返回IPlayer接口,进而调用该接口方法 
         */
         @Override
         public void onServiceConnected(ComponentName name, IBinder service) {
             Log.i("magc", "bind service .....");
             player = IPlayer.Stub.asInterface(service);
             if(player!=null)
             {
                 try {
                     player.setName("magc");
                     player.ToString();
                 } catch (RemoteException e) {
                     // TODO Auto-generated catch block
                     e.printStackTrace();
                 }
             }
         }
     }; 
}
 程序清单:AndroidManifest.xml (注册Activity和Service)
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
       package="com.magc.rpc"
       android:versionCode="1"
       android:versionName="1.0">
     <uses-sdk android:minSdkVersion="9" />
 
    <application android:icon="@drawable/icon" android:label="@string/app_name">
         <activity android:name=".MainActivity"
                   android:label="@string/app_name">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
 
        <service android:name=".MyService">
         <intent-filter>
         <action android:name="com.magc.rpc.action.MYSERVICE" />
         <category android:name="android.intent.category.DEFAULT" />
         </intent-filter>
         </service>
     </application>
 </manifest>

上面Android应用程序运行后结果如下所示:

小结:
1、重点理解Android中对AIDL文件的定义,以及理解ADT工具自动生成的接口类IPlayer,特别是它的静态内部类Stub以及Stub的asInterface方法,
2、Service作为一个代理角色,在其它应用程序通过Stub类的asInterface方法在绑定到一个服务时才能实现返回该接口实例,进而对该实例进行相关操作。

 

posted on 2012-03-10 00:56 vus520 阅读(...) 评论(...) 编辑 收藏

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